Field of the Invention
This invention relates generally to the field of X-ray detector for analytical instruments and, more specifically, to a detector using an indirect conversion screen.
Description of the Related Art
Hybrid pixel array detectors (HPADs) are known in the art and offer near quantum limited performance. In an HPAD, a semiconductor sensor (typically silicon or CdTe) is bump bonded to a readout ASIC (application-specific integrated circuit). Each X-ray that is incident on the detector then produces a charge cloud which is amplified and compared to a binary threshold. If the resultant pulse exceeds the threshold it is counted as an X-ray. If it falls below the threshold it is neglected (assumed to be noise). In this way, most of the detector noise including read noise (also known as kTC noise) and dark current shot noise can be excluded from the final signal. This type of operation is known as “photon counting” and has achieved wide application in a number of areas. However, conventional HPADs have a number of undesirable characteristics which limit their potential application.
Shown in FIG. 1 is a schematic view of a conventional HPAD. In such a configuration, a scintillator screen 10 is positioned on a fiber optic faceplate 12 that couples light emitted from the scintillator to a CMOS sensor 14. The sensor 14 converts the detected light into electrical signals that are conveyed by signal output 16 to an analysis device, such as a computer. The sensor 14 is in contact with a cooling apparatus 18 that maintains it at a relatively low temperature.
Using existing technology, individual sensor-ASIC packages are limited to sizes on the order of a few centimeters or less. This means that a large detector can only be produced by “tiling” a large number of small sensors together with gaps between them. These gaps lead to loss of information which is undesirable or unacceptable in some applications.
Another important limitation of HPADs derives from the use of silicon sensors. For higher energies (above about 12-15 keV) the absorption lengths of X-rays in silicon become large, and thus the signal from X-rays that are incident at an oblique angle are smeared by parallax. Parallax may be avoided through the use semiconductors with higher mean atomic number (Z) such as, for example, cadmium telluride (CdTe) or cadmium zinc telluride (CZT). Indeed, these higher-Z semiconductors are under active development for medical and non-destructive testing applications.
However, for applications in analytical X-ray diffraction, these materials are not ideal since they suffer from polarization which causes the effective conversion efficiency to decrease with time. This drift in quantum efficiency is a crucial disadvantage in X-ray diffraction in which hundreds or even thousands of discrete exposures must be acquired. If the response varies with time, the consolidation of these multiple frames is degraded.